In an exhaust gas purification system provided with an NOx storage reduction catalyst (hereinafter referred to simply as a NOx catalyst) arranged in an exhaust passage of an internal combustion engine, SOx poisoning recovery processing is carried out which serves to reduce the SOx occluded in the NOx catalyst. In the SOx poisoning recovery processing, the air fuel ratio of an exhaust gas flowing into the NOx catalyst (hereinafter referred to as an inflow exhaust gas) is decreased to a predetermined air fuel ratio in a repeated manner. As a result, a reducing agent is supplied to the NOx catalyst and at the same time the temperature of the NOx catalyst rises, so the SOx occluded in the NOx catalyst is reduced.
In Patent Document 1, there is described a technique in which at the time when the air fuel ratio of an inflow exhaust gas is decreased in SOx poisoning recovery processing, the air fuel ratio of the inflow exhaust gas is controlled so that the air fuel ratio of an exhaust gas in an outlet of a NOx catalyst is adjusted to a stoichiometric air fuel ratio.    [Patent Document 1] Japanese patent application laid-open No. 2000-170525